Status

    const Status = packed struct(usize) {
        /// The values of this enum are chosen so that await/cancel can just OR with 0b01 and 0b11
        /// respectively. That *does* clobber `.done`, but that's actually fine, because if the tag
        /// is `.done` then only the awaiter is referencing this `Future` anyway.
        tag: enum(u2) {
            /// The future is queued or running (depending on whether `thread` is set).
            pending = 0b00,
            /// Like `pending`, but the future is being awaited. `Future.awaiter` is populated.
            pending_awaited = 0b01,
            /// Like `pending`, but the future is being canceled. `Future.awaiter` is populated.
            pending_canceled = 0b11,
            /// The future has already completed. `thread` is `.null`, unless the future terminated
            /// with an acknowledged cancel request, in which case `thread` is `.all_ones`.
            done = 0b10,
        },
        /// When the future begins execution, this is atomically updated from `null` to the thread running the
        /// `Future`, so that cancelation knows which thread to cancel.
        thread: Thread.PackedPtr,
    }